Ras protein is a molecular switch in signal transduction pathways that control cell growth and differentiation. In human cells, point mutations in crucial regions of Ras can cause uncontrolled cell growth, or tumors. At least 30% of human cancers show the presence of mutationally activated Ras. Structural studies of Ras-GTP with its effectors and putative effectors will contribute greatly to the understanding of the molecular mechanism of the interaction and Ras inhibitors design. Our studies on Ras and RalGDS will enable us to further understand the structural basis for molecular communication in this system.